1. Field
Systems and methods consistent with the present invention relate to determining air/fuel mass ratio for internal combustion engines, and more particularly to determining air/fuel mass ratio based on current sensed from an engine exhaust gas oxygen sensor.
2. Description of the Related Art
The need for increased control of internal combustion (IC) engines has been an ever-present requirement over the evolution of the IC engine. Over the years, control systems targeted to IC engines have become more sophisticated and complex in order to meet the needs of ever-increasing environmental and operational constraints. One such area of focus relates to maintaining an accurate air/fuel mass ratio (AFR) over all engine operational regions. The AFR is initially determined by measuring incoming air to the engine by a mass air flow sensor or by an intake manifold pressure sensor. The incoming air measurement is then converted to an air flow using the Ideal Gas Law. A precise amount of fuel is added to the air based on the known fuel injector flow rate to achieve the desired AFR. The air-fuel charge is then combusted in the engine and exhausted.
However, there are inaccuracies in the actual AFR that may arise from many sources, and can change during the life of the engine. For example, the airflow sensor can change characteristics due to dirt accumulation, injectors can acquire a varnish coating that changes the actual fuel flow rate, injector spring response can degrade, and fuel characteristics and humidity can vary. Hence, there is a need to obtain an independent measurement of the actual AFR, this measurement being used in a feedback loop that adjusts the injected fuel to more exactly achieve the AFR which is optimal for the engine conditions at hand.
This independent measurement is made after the mix is combusted using an exhaust gas oxygen sensor. These sensors can be of two types: Narrowband (NB) or Wideband (WB). An NB oxygen sensor simply determines if the exhaust gas is lean (excess oxygen) or rich (all oxygen is bound to another element). It provides a voltage that is sent directly to an engine control unit (ECU) which then adjusts fuel to maintain a stoichiometric ratio. This voltage from the NB sensor indicates that all free oxygen in the exhaust has been consumed and there is no excess CO or H2.
A WB oxygen sensor, also known as a universal exhaust gas oxygen (UEGO) sensor, provides a measure of the degree of richness and leanness of the air/fuel ratio. This type of WB sensor provides an increased sensor signal bandwidth to the ECU in order to maintain stoichiometric operation and optimum catalyst efficiency. In motorsports applications, the UEGO sensor signal provides a direct feedback on the air/fuel ratio (AFR) which can be tailored for a desired mixture.
Another important use of the UEGO sensor signal is to accommodate different fuel mixes. Until recently, the vast majority of passenger cars have used only gasoline as a fuel. However, with fossil fuel resources being depleted, many alternative fuels are being pressed into service. These fuels include the 10% ethanol/90% gasoline mix now commonly sold at most filling stations, and an 85% ethanol mix (E85) that is also becoming popular. These fuels result in a significantly different AFR, both on and off the stoichiometric point. For example pure ethanol has a stoichiometric AFR of 9.0, while the stoichiometric AFR for regular gasoline is 14.7. It is for this reason that OEMs introduced special Flex-Fuel sensors in the fuel delivery system to provide a signal indicating the percentage gas/alcohol mix. Later model automobiles use UEGO signals directly in order to determine fuel composition mixture in so-called virtual-sensor arrangements.
In motorsport applications the use of alternative fuels is very prevalent—hydrocarbons such as diesel, ethanol, and nitro-methane are common. Oxidizers such as nitrous oxide are also very common. Additionally, water injection is used to reduce the tendency of detonation in boost applications. And all of the above can be combined all at once in various proportions depending on operating situation. The effect combining fuels is an altering of the final hydrocarbon and oxygen/nitrogen the engine ultimately has to ignite for combustion. And since this fuel combination determines the exhaust gas species molar concentration, and hence UEGO sensor pump current (to maintain Nernst cell stoichiometry within the sensor head), it is important to take the fuel composition into account. Expensive, lab-grade UEGO sensor calibration meters allow the entry of hydrocarbon H/C and O/C ratios and humidity/vapor pressure, but these are fixed quantities and not adjustable in real-time. Lower-cost UEGO sensor controllers do not offer any adjustment in fuel composition, assuming a fixed H/C and O/C fuel for all conditions.
Existing aftermarket wideband UEGO controllers do not have provisions for alternative hydrocarbon fuels or additional sources of oxidizers such as N20. While the lambda value i.e., the normalized air/fuel ration relative to the stoichiometric point is a fair approximation for many fuels, it is not exact because the sensitivities of the UEGO sensor to CO and H2 have not been taken into account on the rich side of the stoichiometric point, and the error grows as the AFR departs from stoichiometric on both the lean side and even more on the rich side.
Also, the lambda value is generally less intuitive to users of the WB controller for motorsport applications, who are more familiar with the AFR value. While the lambda value can be converted to an AFR, it requires the user to input the stoichiometric AFR for the fuel being used. While this can easily be done for pure fuels such as pure gasoline and ethanol, it needs to be calculated for fuel/oxidizer mixes. For varying mixes, for example, when a user has a partial tank of gasoline and fills up with E85, calculation of the stoichiometric AFR becomes even more problematic. Existing aftermarket UEGO sensor controllers do not address this problem, nor do they allow the entry/ implementation of real-time hydrocarbon/oxidizer/H20 mixtures.
Another limiting aspect with current UEGO sensor controllers available is the lack of UEGO sensor calibration. The controllers only allow, at best, calibration in free air, assuming an O2 content of 20.9% and extrapolating this value for both lean and rich lambda calculation. Other UEGO sensor controllers use a fixed published lambda-vs-pump current transfer function, valid only for a predetermined gas combination and UEGO sensor. Free-air calibration will yield a satisfactory calibration for lean side of stoichiometric (i.e., excess oxygen, lambda>1), however rich-side operation is not sufficiently calibrated. In fuel-rich combustion where all oxygen is consumed there are many gas species remaining, including CO, H2 and unburned HC. The UEGO sensor operates by reducing the CO and H2 into CO2 and H2O (i.e., CO+O2→2CO2 and H2+O2→2H2O).
It is apparent that a calibration utilizing oxygen-only will not adequately determine H2 and CO sensor sensitivities. It should be noted that in motorsports applications, fuel-rich operation is often desirable since optimum engine torque output often occurs at regions around lambda=0.9. In this case, maintaining stoichiometric operation is not a requirement.
Finally, the actual determination of lambda and AFR from UEGO pump current readings is not well-defined in aftermarket controllers. As stated earlier, many controllers utilize default pump current vs. lambda values from sensor-head manufacturer data. Of course, the real world requires the use of sensor calibration along with knowledge of the hydrocarbon under combustion-both are required for accurate AFR and lambda determination.
Sensor-head calibration is easily accomplished by bench-testing UEGO sensors with known gas compositions, namely O2, CO and H2. It should be noted that although the UEGO sensor is also sensitive to unburned hydrocarbons, the amount of such constituents in the exhaust of a modem engine operating under normal operating conditions is in the parts-per-million range, whereas the concentrations of CO and H2 are orders of magnitude higher.
The proper way to determine lambda and AFR from a known hydrocarbon fuel is by chemical balance equations. In simple terms, the balance equation keeps track of the moles (or concentrations or partial pressures) of each gas species before and after combustion—nothing is lost and all components have to be accounted for. This calculation can be complicated, and has been the topic of several technical papers. Most are targeted for situations where each specific gas component is individually measured, as is the case with 4 and 5-gas bench analyzers.
For instance a method of calculating lambda value is described in the paper of J. Brettschneider, “Calculation of the air ratio of air-fuel mixtures and the influence of measurement errors on lambda” in Bosch Technische Berichte, Bd. 6, Heft 4 (1979), pp. 177 to 186. The Brettschneider calculation has become the standard for lambda calculation in multi-gas analyzers, although there are comparable calculations presented in the literature by Silva, Spindt, and Simons. The approaches outlined in each of these papers have the luxury of separate and independent measurement of each of the gas constituents. In other words, there are separate sensor elements which are sensitive to gas components O2, CO, etc. In a UEGO sensor, on the other hand, there is one sensor element that is responsive to multiple gas components, and hence one measurement. In algebraic terms, there are multiple equations (chemical balance equations) with unknowns, but only one measurement. Hence, relations that yield more information from the one measurement source are needed. These relations come from the knowledge of the hydrocarbon under combustion and the UEGO sensor element sensitivities to specific gas constituents.
Simplifications have been suggested to reduce the computational burden in order to provide real-time updates, generally in the form of scaling fixed response curves or transfer functions obtained in the most general case. This leads to a simplified lambda calculation for fuel-rich combustion and works sufficiently well when the hydrocarbon is fixed, as is the case in a production engine. However, when the fuel types and proportions change, the sensitivities become important and influence the calculation directly.